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First academic paper presents New Horizons Pluto findings: pits and troughs found on Tombaugh Reggio

Pluto in color (cropped).

This high-resolution image captured by NASA’s New Horizons spacecraft combines blue, red, and infrared images taken by the Ralph/Multispectral Visual Imaging Camera (MVIC). The bright expanse is the western lobe of the “heart”, informally called Sputnik Planum, which has been found to be rich in nitrogen, carbon monoxide, and methane ices. Image & Caption Credit: NASA / JHU-APL / SwRI

The New Horizons team has published an eight-page academic paper in the October 16 issue of the journal Science. In the paper, the team discusses the New Horizons spacecraft’s diverse findings at the Pluto system during its historic July 14, 2015, flyby.

Titled The Pluto System: Initial Results from its Exploration by New Horizons, and led by mission Principal Investigator Alan Stern, it is the first peer-reviewed paper to discuss geological activity on Pluto and Charon, the composition of Pluto’s atmosphere, and the many images sent back by the science instruments on the spacecraft.

Alongside the paper, which is featured as the publication’s cover story, is a new, enhanced image (above) of Pluto that emphasizes the wide range of colors across the dwarf planet’s surface.

The Rich Color Variations of Pluto

Pluto. (Click to enlarge.) Photo Credit: NASA / JHU-APL / SwRI

Clearly visible in the images (above and on the right) are dark red regions at Pluto’s equator, blue areas at higher latitudes, and the iconic heart known as Tombaugh Reggio, which itself displays different colors at its eastern and western sections.

“We knew Pluto’s surface was heterogeneous based on ground-based data. However, I was astonished to see such spectacular surface color and geological diversity,” said University of Maryland astronomer Silvia Protopapa, one of the paper’s many authors.

Protopapa and her colleagues studied data returned by the Linear Etalon Imaging Spectral Array (LEISA), a near-infrared spectral imager that mapped the composition of Pluto and its moons, to determine whether the surface color differences are caused by varied surface composition.

The team did find regional differences in terms of surface volatile ices. The dark red regions at Pluto’s equator contain very little volatile ice, whereas the western section of the heart is composed of both methane ice and carbon monoxide ice.

Even more baffling than the variety of terrains, colors, composition, and reflectivity is the fact that both Pluto and Charon appear to be geologically active worlds.

“The Pluto system surprised us in many ways, most notably teaching us that small planets can remain active billions of years after their formation,” Alan Stern, principal investigator and lead author, said. “We were also taught important lessons by the degree of geological complexity that both Pluto and its large moon Charon display.”

Pluto’s water-ice bedrock and the mobile ices in the western section of Tombaugh Reggio indicate the small planet is still geologically active today.

“Pluto’s still got an engine, and it’s still running,” Stern said.

The surprising lack of craters in the section of the heart known informally as Sputnik Planum shows that this area is being continually reshaped, possibly by tectonic forces. One theory attributes these forces to internal heating produced through radioactive decay of rocky material in Pluto’s core.

Puzzling pits on Pluto

This image was taken by the Long Range Reconnaissance Imager (LORRI) on NASA’s New Horizons spacecraft shortly before closest approach to Pluto on July 14, 2015; it resolves details as small as 270 yards (250 meters). The scene shown is about 130 miles (210 km) across. The Sun illuminates the scene from the left, and north is to the upper left. Image & Caption Credit: NASA / JHU-APL / SwRI

On the day the paper was published, the mission team released a new close-up image of Sputnik Planum showing clusters of pits and troughs as well as the plain’s strange cellular pattern.

Taken by the Long Range Reconnaissance Imager (LORRI) just before closest approach, the image (above) resolves features as small as 270 yards (250 meters) and covers an area 130 miles (210 km) across.

Close-up Pluto's moon Charon

This mosaic of Pluto’s largest moon Charon was taken by the Long Range Reconnaissance Imager (LORRI) on New Horizons shortly before closest approach on July 14, 2015; it resolves details as small as 340 yards (310 meters). The scene at the bottom is about 125 miles (200 km) across. (Click to enlarge.) Image & Caption Credit: NASA / JHU-APL / SwRI

Patterned clusters of small pits and troughs are visible from the lower left to the upper right of the image. Sputnik Planum’s puzzling cellular patterns cover a significant area on the photo’s left side.

Knowing Sputnik Planum is an area of volatile ices such as solid nitrogen, mission scientists speculate the pits and troughs are created by these ices’ sublimation or evaporation. The pits and troughs are estimated to be tens of meters deep and hundreds of meters in diameter.

As of now, scientists have no explanation for these features bizarre alignments and shapes, adding yet another mystery to the growing Pluto puzzle.

Even at this close range, there are no visible impact craters, meaning the terrain is geologically young.

Project scientist Hal Weaver of Johns Hopkins University Applied Physics Lab (JHU-APL) believes the alignment of the pits and troughs could provide clues about the flow of ice on Sputnik Planum and the process by which volatiles are exchanged between the planet’s surface and atmosphere.

“The science team is working hard to understand what physical processes are at play here,” he said.

The Science paper also highlights Pluto’s 11,000-foot (3.353 km) high mountains of water ice and red areas likely colored by the presence of tholins, produced by the interaction of methane and nitrogen with ultraviolet sunlight.

Scientists remain puzzled as to how small worlds so far from the Sun can still be geologically active today. Activity on Pluto and Charon raises questions about the possibility of similar geological processes taking place on other remote dwarf planets such as Haumea, Makemake, and Eris.

The paper notes Pluto’s thin, multiple haze layers that stretch 90 miles (144.8 km) from its surface and are composed of nitrogen and other gases. While the dwarf planet’s atmospheric pressure is lower than expected, it is premature to conclude that the atmosphere is freezing out as Pluto recedes from the Sun after its 1989 perihelion.

NASA's New Horizons captured this high-resolution enhanced color view of Charon just before closest approach on July 14, 2015.

Charon. (Click to enlarge.) Photo Credit: NASA / JHU-APL / SwRI

New Horizons provided an accurate measure of Pluto’s size, finding it slightly larger and somewhat icier than expected.

For many years, Pluto was believed to have surface features and composition similar to Neptune’s large moon Triton, but the flyby data has shown that the geology of the two objects is very different, according to the paper.

The article goes on to describe Charon, whose surface is significantly different than that of Pluto, featuring a system of deep canyons and a dark north polar region whose reddish color may be caused by the capture of volatile materials escaping from Pluto that are then turned into tholins.

New, sweeping mosaics of Charon (shown above left), made with LORRI’s highest resolution photos of the large moon, were also released on October 16, 2015.

The mosaic at the top covers a large area stretching from the limb to the terminator; the lower one covers a region 125 miles (210 km) in diameter. The close-up images used in the mosaics resolve features as small as 310 meters (340 yards).

Uneven, cratered terrain is visible on the left side of the second mosaic, which features the informally named Vulcan Planum. Eventually, this terrain gives way to large canyons in the region informally named Serenity Chasma. Beyond Serenity and toward the right of the image is younger, resurfaced terrain.

Some sections of Vulcan Planum contain irregularly spaced grooves and craters, somewhat resembling the lunar mare, volcanic plains on Earth’s moon. Unlike the Moon’s mare regions, which are composed of basalt, Charon’s plains are made of water ice.

Two of the system’s four small moons discussed in the paper – Nix and Hydra – are also composed largely of water ice.

New close-up images of Nix and Hydra were released by the mission team the previous week. Both have diameters less than 30 miles (48.3 km). Bright Nix shows a large central crater that has a reddish color, likely the result of an early impact that excavated darker material from beneath the moon’s surface.

Hydra appears to be composed of two lobes, much like Comet 67P/Churyumov–Gerasimenko, meaning it could be the result of a merger between two initially separate objects.

Pluto's moons Nix and Hydra

On the left, Pluto’s moon Nix is shown in high-resolution black-and-white and lower resolution color. On the right, Pluto’s moon Hydra. (Click to enlarge.) Image Credit: NASA / JHU-APL / SwRI

An image of Kerberos has not yet been sent back by the spacecraft; however, a blurry image of Styx was received by the mission team last week.

The small moons spin very rapidly and tumble in their orbits around the center of gravity between Pluto and Charon, the article notes.

Mark Sykes of the Planetary Science Institute, who is not a member of the New Horizons team, but did review the paper, acknowledged the Pluto system is far more diverse and dynamic than any scientists had thought before the flyby.

He also noted the paper’s authors refer to Pluto and similarly-sized objects as planets, counter to the practice of the International Astronomical Union (IAU).

The paper is available for free download at



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

Reader Comments

Hmmm, Sputnik Planum looks very familiar. Looks like cooling foam on top of hot, thick soup. The bubbles pop but leave depressions as the solids congeal in the cooler air above. Then there’s the evidence of thermals underneath, welling up and segmenting the surface into gentle domes.

By God, They’ve discovered white gravy cooking on Pluto!

–Jim B.

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